Increasingly, optical network designers have sought to determine the effects of environmental conditions on the performance of network equipment. The results suggest that the effects are sizable.
Employed in systems from local area networks (LAN) to nationwide communication networks, optical networks are a preferred way to achieve reliable, low-cost data transmission. Though designs vary depending on size, complexity and other factors, these networks are generally a complex system of interconnected optical components that perform signal input/output, switching, data processing, data analysis, and other functions.
Increasingly, as networks grow in size, optical components are placed in varied locations, some with climate control many without. Unfortunately, temperature variations affect the performance of many devices. Affected devices may introduce substantial loss into a network due to signal degradation and, in a worst-case scenario, may result in malfunction of the entire network. Many devices, such as arrayed waveguide gratings (AWGs), are susceptible to this detrimental temperature dependence.
Commonly, optical networks are wavelength division multiplexing (WDM) systems that transmit multiple data streams simultaneously as data channels, each channel being centered at a different wavelength or carrier frequency. In WDM systems, multiplexing and demultiplexing functions (i.e., the processes of combining and parsing channels) are often performed by AWGs.
An AWG generally includes an input waveguide and an array of output waveguides—each of varying optical path length—that produce an output pattern spatially separating-out the channels in the input signal. This channel separation depends upon the phase differences in the waveguide array, which makes AWGs very sensitive to environmental effects.
In attempting to control for environmental conditions, active temperature compensation in the form of thermostating devices has been used. Undesirably large amounts of power are required to run these devices, however. A few devices have been proposed with passive thermal compensation (e.g., structures that use thin film filters or regular gratings with bulk optics and structures with negative thermo-optic coefficient materials). Yet, these are expensive to fabricate and have inherent losses (both polarization independent and dependent loss). Also, their operative temperature ranges are limited due to linearity and temperature dependent losses. Reflection losses also limit device performance.
Other approaches include devices that control movement of a fiber support arm connected directly to an optical fiber. The fiber is moved by the support in response to temperature changes. These approaches, however, have numerous problems.
Accurately mounting and positioning the fiber support arm to the fiber is difficult. Further, fine-tuning support arm movement often requires complex structures, and, even with these, the fine-movement required is still not readily achievable. Also, there is nothing to prevent the support arm from moving vertically out of the plane of the AWG. Additionally, fabrication costs are high and stiction is a problem. Device scalability is also limited. And, for these designs and all of the above-described designs, the inherent losses are so great that larger-scale AWGs are not feasible.